Method for the reduction of zinc ion concentration and removal of organic impurities in a neutral or acidic, aqueous zinc plating bath

ABSTRACT

A composition and method for the reduction of zinc ion concentration and removal of organic impurities in a neutral or acidic, aqueous zinc electroplating bath by adding directly to the bath ammonium oxalate, or the combination of oxalic acid and a stoichiometric amount of ammonium hydroxide.

United States Patent [191 [111 3,887,445

Jackson V v June 3, 1975 METHOD FOR THE REDUCTION OF ZINC [56] References Cited ION CONCENTRATION AND REMOVAL OF ORGANIC IMPURITIES IN A NEUTRAL OR ACIDIC, AQUEOUS ZINC PLATING BATH Inventor: Joseph L. Jackson, Westlake, Ohio Assignee: R. 0. Hull & Company Incorporated, Cleveland, Ohio Filed: Apr. 26, 1974 Appl. No.: 465,000

US. Cl 204/55 R; 2l0/54 Int. Cl. C23b 5/10; C23b 5/12; BOld 9/00 Field of Search 204/55 R, 43 Z, 44, 114,

OTHER PUBLICATIONS Frederick A. Lowenheim, Modern Electro-Plating pp. 400-401, (1968).

Primary ExaminerG. L. Kaplan Attorney, Agent, or Firm-Oldham & Oldham Co.

[57] ABSTRACT 5 Claims, No Drawings 1 METHOD FOR THE REDUCTION OF ZINC ON CONCENTRATION AND REMOVALOF ORG NIC IMPURITIES I A NEUTRAL R ACIDIC, AQUEOUS ZINC PLATING BATH BACKGROUND OF THE INVENTION In the operation .of an acid-or neutral zinc electroplating bath, the cathode efficiency is frequently lower than the anode efficiency which results in the build up of the zinc ion concentration. In other words, the anodes dissolve faster than the zinc plates out of the bath. The build up of zinc ionis undesirable from' the standpoint of poorer metal distribution on the cathode and a less bright plate in areas of lower current density. In addition, the higher metal content increases the problems of adequate rinsing and results in a higher zinc content in the effluent'which is a pollutant.

A reduction in zinc content by discarding a portion of the bath results in a disposal problem as well as being costly in requiring the addition of other necessary or beneficial components of the bath make-up for the losses due to dilution.

SUMMARY OFTHE INVENTION It is therefore, an object of the present invention to reduce and/or prevent the build up of zinc ion in an acid or neutral zinc electroplating bath.

It is another object of the present invention to add specific compounds to a neutral or acid zinc electroplating bath to precipitate excess amounts of undesirable zinc ions. I

It is still another Object of the present invention to precipitate excess zinc ions, as above, by adding either ammonium oxalate or oxalic acid and an equivalent amount of ammonium hydroxide.

It is a further object of the present invention to pro vide a method of reducing the amount of zinc ion concentrate as well as the removal of organic impurities in a neutral or acidic aqueous zinc electroplatin'g'bath by adding ammonium oxalate or oxalic acid and an equivalent amount of ammonium hydroxide to the plating bath and removing the ensuing precipitate which is composed of zinc oxalate and absorbed organic impurities.

It is a still further Object of the present invention to remove excess zinc ions in an acid or neutral zinc electroplating bath which is efficient, non expensive, and in a non polluting manner.

These and other objects of the present invention, together with the advantages thereof over existing prior art compounds and methods which will become apparent from the following specification are accomplished by the compounds and methods herein described and claimed.

In general, a method for the reduction of zinc ion concentration in a neutral or acid aqueous zinc electroplating bath comprises the steps of, adding to the bath a compound selected from the group consisting of ammonium oxalate, and the combination of oxalic acid, and a stoichiometric amount of ammonium hydroxide, to form a precipitate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the concepts of the present invention the precipitation of excess zinc ions from a neutral or acidic zinc electroplating bath is readily accomplished by directly adding to the bath ammonium oxalate, or the combination of oxalic acid and a stoichiometric amount of ammonium hydroxide.

Generally the amount of ammonium oxalate, or the combinationof oxalic acid and the stoichiometric amount of ammonium hydroxide depends upon the amount of excess Zinc ions. That is, if a large zinc ion build up exists, a larger amount of the compounds according to the present invention will be required to remove this excess than for example if a smaller excess of zinc ions exists. Additionally, the amount of compounds according to the present invention will also vary dependingupon the extent of desired removal of the excess zinc ions. Thus, the actual amount of ammonium oxide or oxalic acid will tend to vary over a broad range and can be readily determined by one skilled in the art. Concerning the ammonium hydroxide, since it is desirably a stoichiometric amount based upon the oxalic acid, the amount utilized is generally about 2.0 equivalents.

The above noted chemical reactions are described as follows:

those skilled in the art.

TABLE I Ammonium Chloride Zinc Electroplating Bath INGREDIENT CONCENTRATION Zinc Chloride 30to g/l Ammonium Chloride I00 to 300 g/l pH 4 to 7.5

It is desirable to maintain the ammonium chloride in the bath in Table I within the approximate range of 100 to 300 g/l. To lower the zinc content ofa bath by simple dilution would also cause unwanted reduction in the ammonium chloride concentration. It is then very costly and time-consuming to replace the ammonium chloride. With the method of the present invention the zinc ion is removed and simply replaced by ammonium ion, thus eliminating the necessity for other undesirable bath adjustments. Examples I and II illustrate the simplicity of this method for decreasing the zinc in concentration.

EXAMPLE I To 1 liter of solution containing 31.5 g/l of zinc at a pH of 6.07 and g/l of chloride, 32 grams of oxalic acid was added. The pH dropped slowly while mixing during the first 5 minutes to 4.12, then it fell rapidly in the next minute to 1.35 and stabilized at 0.66 after a total time of 18 minutes. The solution required 33.4 ml. of concentrated ammonium hydroxide (28% NH;) to

return the pH to 6.07. After decanting, the analysis was 14.2 g/l of zinc and the chloride remained at about 150 g/l. When adjusting for the slight increase in volume due to the ammonia, the reaction was stoichiometric in accordance with the equation;

EXAMPLE II The same solution was used as in Example I but the oxalate ion was added in the form of 36 grams of ammonium oxalate. The reaction much slower as indicated by periodic analysis for zinc. After about 2 /2 hours of vigorous stirring, the pH was lowered to the staring pH by adding small amount of hydrochloric acid. After decanting, the zinc content was found to be 14.2 g/l. This reaction was stoichiometric according to the equation;

As apparent from the above stoichiometric equations, the desirable amounts of compounds required can be readily determined once the excess amount of zinc ion (Zn is known and the desirable amount of zinc ion for removal is determined.

An unexpected benefit of the reduction in zinc ions is that organic impurities are also removed by being absorbed on the surface of the zinc oxalate precipitate. These organic impurities are generally electrolytic break-down products of the brightening agents, but can also be oils and greases that enter the bath on parts to be plated that havent been thoroughly cleaned. These organic impurities cause smutty, stained, and dark electrodeposits and a common method of removing them is by treating with activated carbon. The carbon treatment is expensive and very time consuming, since it requires that the bath be carefully filtered.

When the bath is treated with ammonium oxalate, or the combination of oxalic acid and ammonium hydroxide, the heavy zinc oxalate precipitated along with the organic impurities falls quickly to the bottom of the treatment tank. The bath is simply decanted instead of filtered and the precipitate is discarded.

The concepts of the present invention and the compounds and methods utilized therein have been explained in detail with respect to the preferred embodiments. As apparent to one skilled in the art, various modifications can be made according to the concepts of the present invention without departing from the spirit of the invention herein disclosed. While in accordance with the Patent Statutes, the preferred embodiments have been illustrated and described in detail, it is to be understood that the invention is not limited thereto; the scope of the invention being measured solely by the scope of the attached claims.

What is claimed is:

1. A method for the reduction of excess zinc ion concentration in an aqueous acid or neutral zinc electroplating bath comprising the steps of, adding to the bath a compound selected from the group consisting of ammonium oxalate, and the combination of oxalic acid and (an amount) about 2.0 equivalents of ammonium hydroxide based on said oxalic acid, to form a zinc precipitate.

2. A method for the reduction of excess zinc ion concentrate according to claim 1, including the additional step of removing said precipitate.

3. A method for the reduction of excess zinc ion concentration according to claim 2 wherein organic impurities are additionally precipitated and removed by said added bath compound, said organic impurities selected from the class consisting of electrolytic break-down products of brightening compounds, oils and greases.

4. A method for the reduction of excess zinc ion concentration according to claim 1, wherein the bath is an aqueous ammonium chloride zinc electroplating bath, said bath containing from 30 to 120 grams per liter of zinc chloride and from to 300 grams per liter of ammonium chloride.

5. A method for the reduction of excess zinc ion concentration in an aqueous acid or neutral zinc electroplating bath according to claim 1, wherein said amount of ammonium hydroxide is stoichiometric in relation to said oxalic acid. 

1. A method for the reduction of excess zinc ion concentration in an aqueous acid or neutral zinc electroplating bath comprising the steps of, adding to the bath a compound selected from the group consisting of ammonium oxalate, and the combination of oxalic acid and (an amount) about 2.0 equivalents of ammonium hydroxide based on said oxalic acid, to form a zinc precipitate.
 1. A METHOD FOR THE REDUCTION OF EXCESS ZINC ION CONCENTRATION IN AN AQUEOUS ACID OR NUETRAL ZINC ELECTROPLATING BATH COMPRISING THE STEPS OF, ADDING TO THE BATH A COMPOUND SELECTED FROM THE GROUP CONSISTING OF AMMONIUM OXALATE, AND THE COMBINATION OF OXALIC ACID AND (AN AMOUNT) ABOUT 2.0 EQUIVALENTS OF AMMONIUM HYDROXIDE BASED ON SAID OXALIC ACID TO FORM A ZINC PRECITATE.
 2. A method for the reduction of excess zinc ion concentrate according to claim 1, including the additional step of removing said precipitate.
 3. A method for the reduction of excess zinc ion concentration according to claim 2 wherein organic impurities are additionally precipitated and removed by said added bath compound, said organic impurities selected from the class consisting of electrolytic break-down products of brightening compounds, oils and greases.
 4. A method for the reduction of excess zinc ion concentration according to claim 1, wherein the bath is an aqueous ammonium chloride zinc electroplating bath, said bath containing from 30 to 120 grams per liter of zinc chloride and from 100 to 300 grams per liter of ammonium chloride. 